# Thread: Derating calculation problem

1. Originally Posted by mohsan514
I have chiller units and each unit is 600 amps and required OCPD is 800amp. I used the 2 runs of cable and total ampacity is 820 amp and these are laid in perforated cable tray . After applying grouping factor / correction factor , the cable ampacity is reduced to 720 amps. But i am worried that how 800 amp CB will protect the conductor in case of any abnormal condition.
Since nec 240.4 allows to use the higher rated OCPD. Even the conductor is less rated..
Please advise

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Motors (and refrigeration units) have higher branch circuit devices than conductor ampacity quite often, this to allow for overcurrent device to hold during starting. There is still motor overload protection that will limit current to conductor ampacity or only allow limited overloading before it interrupts the circuit.

Take a simple 10 Hp 480 volt motor - FLA may be about 12 amps, so overload protection is set according to 12 amps (probably going to be 115% to 125% of that most cases but based on that 12 amps) A branch circuit breaker could possibly be as high as 35 amps. You are still allowed 14 AWG conductor because the motor overload protection will never allow 14 AWG to be overloaded for long enough to cause any damage to the conductor.

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I am looking for correction factor or de-rating factor of touching single core cables installed in trefoil arrangement at cable tray . 3 ckt at each cable tray. Please advise with code reference.
Thanks

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3. Originally Posted by mohsan514
I am looking for correction factor or de-rating factor of touching single core cables installed in trefoil arrangement at cable tray . 3 ckt at each cable tray. Please advise with code reference.
Thanks

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Is this what you are looking for @mohsan514

Table 392.22(A)(5) Allowable Cable Fill Area for
Multiconductor Cables in Ventilated Channel Cable Trays for
Cables Rated 2000 Volts or Less

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Thank you.. same i was looking for..

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